In many parts of the St. Lawrence drainage system, enormous numbers of Trichoptera, often mingled with Ephemerida, emerge during the summer and become a pest in a considerable zone bordering the water. The insects swarm in open areas, especially around trees and lights, invade houses, settle in large numbers on walls and other surfaces, ruining those that have been freshly painted, and pile in decaying and odoriferous drifts along the shore line. In addition, they give rise to a medical problem by evoking allergic reactions in hypersensitive individuals.